Re: [RFC PATCH 1/3] memory tiering: read last_cpupid correctly in do_huge_pmd_numa_page()

[Huang, Ying]

Zi Yan <zi.yan@xxxxxxxx> writes:

> From: Zi Yan <ziy@xxxxxxxxxx>
>
> last_cpupid is only available when memory tiering is off or the folio
> is in toptier node. Complete the check to read last_cpupid when it is
> available.
>
> Before the fix, the default last_cpupid will be used even if memory
> tiering mode is turned off at runtime instead of the actual value. This
> can prevent task_numa_fault() from getting right numa fault stats, but
> should not cause any crash. User might see performance changes after the
> fix.
>
> Fixes: 33024536bafd ("memory tiering: hot page selection with hint page fault latency")
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>

Good catch!  Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

> ---
>  mm/huge_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d7c84480f1a4..07d9dde4ca33 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1705,7 +1705,8 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>  	 * For memory tiering mode, cpupid of slow memory page is used
>  	 * to record page access time.  So use default value.
>  	 */
> -	if (node_is_toptier(nid))
> +	if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) ||
> +	    node_is_toptier(nid))
>  		last_cpupid = folio_last_cpupid(folio);
>  	target_nid = numa_migrate_prep(folio, vmf, haddr, nid, &flags);
>  	if (target_nid == NUMA_NO_NODE)